Ammoniacal levels (often referred to as "ammonia") are found normally in the body and ordinarily are not harmful, yet in increased concentration become toxic. Hyperammonemia is the clinical condition associated with increased plasma ammoniac levels which manifests itself in vomiting, lethargy, confusion, and coma. Prognosis for patients suffering from hyperammonemia depends on prompt detection and aggressive treatment. Once it has been recognized that a patient is suffering from hyperammonemia, there are alternatives available for lowering the level of ammoniac component present in the blood. If undetected or untreated, however, continuing hyperammonemia may result in severe brain damage or death.
Hyperammonemia is not a diagnosis, rather it is a condition which may result from one of any number of underlying causes which range from inherited abnormalities, to acquired diseases, to inducement during the course of treatment for other illnesses. The normal ammoniacal concentration ranges from 15 to 35 .mu.mol/liter in adults and 20 to 50 .mu.mol/liter in children. A patient may experience a symptomatic range including vomiting, loss of muscle coordination, irritability and hyperactivity at 100 or above .mu.mol/liter, vomiting and lethargy at 200 .mu.mol/liter, and coma at or above 300 .mu.mol/liter. While these ammoniacal concentration levels may seem high, being double to six times the normal levels in a healthy adult, ammoniacal concentration levels for inherited disorders have been reported being over 1000 .mu.mol/liter to as much as 4000 .mu.mol/liter.
The highest levels of ammoniacal concentration are reported in cases of transient hyperammonemia where concentration may rise to 2000 to 4000 .mu.mol/liter, nearly 100 times greater than normal. This occurs with one type of transient hyperammonemia whose cause, while still uncertain, has been linked to transient abnormalities of the urea cycle, delayed development of an affecting enzyme outside the urea cycle, tissue hypoxia or poor perfusion through the liver. Another type of transient hyperammonemia involves ammoniacal concentration levels which are approximately twice the normal level, but which generally decreases to normal without treatment.
Inherited disorders of the urea cycle also may cause hyperammonemia in both adults and children, although the most severely affected are present in the neonatal period. If there is a deficiency in one of the urea cycle enzymes, inadequate urea will be formed and nitrogen, in the form of an ammoniacal concentration, will accumulate in all cells of the body. Congenital deficiencies of each of the five enzymes in the urea cycle have been identified. In children, high levels of ammoniac concentration often will manifest itself as a catastrophic illness known as hyperammonemic coma. Morbidity has been associated with the duration of hyperammonemic coma rather than with the specific enzyme deficiency causing the level of ammoniacal concentration elevation.
Another inherited disorder associated with hyperammonemia is organic acidemias, which is a defect in the metabolism of amino acids and fatty acids. A metabolic crisis may be precipitated by excessive protein intake, intercurrent infections, incorrect diet or incorrect medications. For more information on hyperammonemia caused by inherited disorders, see:
1. Ballard, R. A., et al. "Transient Hyperammonemia of the Preterm Infant." New England Journal of Medicine. 1978; 299: 920-925. PA1 2. Batshaw, M. L., et al. "Treatment of urea Cycle Disorders." Enzyme. 1987; 38: 242-250. PA1 3. Leonard, J. V. "Hyperammonemia in Childhood." Clayton, B. E., ed. Chemical Pathology and the Sick Child Oxford: Blackwell, 1984: 96-119. PA1 4. Burtis, C. A. and E. R. Ashwood, eds. Teitz Textbook of Clinical Chemistry (second edition). Philadelphia: W. B. Saunders Company, 1994. pp. 1487-1489. PA1 5. losefoshn, M. "Ektachem Multilayer Dry-Film Assay for Ammonia Evaluation." Clinical Chemistry. 1985; 31 (12): 2012-2014. PA1 6. Quiles, R., et al. "Continuous flow assay of Ammonia in Plasma Using Immobilized Enzymes." Analytica Chimica Acta 1994; 294 (1): 43-47.
In addition to inherent abnormalities, hyperammonemia may be caused by acquired diseases or conditions. The leading cause of hyperammonemia in adults is intrinsic liver disease. Acute liver disease being caused by viral hepatitis, drug overdose, reaction to anesthetic agents or medications, and obstruction of bile duct, while the most common causes of chronic liver disease in adults include cirrhosis, infection, excessive protein intake, diuresis, and sedative drugs. Renal failure can precipitate or exacerbate hepatic encephalopathy by excessive production of ammonia. Other diseases or conditions, such as leukemia, urinary tract infections, congestive heart failure, physical trauma to the liver or kidneys, or disseminated herpes simplex infection also may cause hyperammonemia.
A final category of causes for hyperammonemia is inducement during treatment for other illnesses. Sodium valproate is an anti-epileptic agent used to control generalized seizures and other refractory types of seizures which has been reported to cause high levels of ammoniacal concentration in the blood. Hemodialysis may lower ammoniacal concentration levels in patients with hepatic encephalopathy, however, the opposite may be found during hemodialysis with sorbent-based low-volume dialysate regeneration systems. With these systems, urea is converted to ammoniacal components which then are absorbed by a cationic exchange resin. If the absorption rate of the resin is exceeded, these components continue to be converted but diffuses from the dialysate into the patient. Hyperammonemia is also a risk during transurethral resection of the prostate using glycine irrigant due to the metabolic decomposition of glycine into ammoniacal components. Heart and lung transplantation may be accompanied by hyperammonemia, which if not promptly and aggressively treated, can be a life threatening complication.
While the foregoing is not an exhaustive list of potential causes of hyperammonemia, these examples illustrate the wide variety of sources of increased ammoniacal concentration levels and the seriousness of the resulting condition. Fortunately, once a hyperammonemic episode has been identified, a number of intervention alternatives are available to lower ammonia levels. For example, in urea cycle disorders these include limiting nitrogen intake, improving the quality of protein ingested, supplying deficient metabolites, providing alternate pathways for waste nitrogen excretion and removal of nitrogen, i.e., by peritoneal dialysis or hemodialysis. Cases of acute hyperammonemia may require mannitol infusions to control intracranial pressure. With ammoniacal concentration levels decreased to within acceptable bounds, the underlying cause may be addressed.
Several conventional methods currently are available to measure the ammoniacal concentration level present in a patient. Most of these require some form of separation process before analysis. Ammonia gas and ammonium ion are separated from their matrix either by absorption onto a resin or by conversion to ammonia using alkali followed by gaseous diffusion. The ammonia gas concentration may then be quantified colorimetrically or by an ion-specific electrode. Alternatively, enzymatic methods are available which involve the formation of reaction product, proportional to the presence of ammonium ion, which is measured spectrophotometrically or fluorimetrically. While these methods may measure ammoniacal concentration levels with a certain degree of accuracy if performed properly, there are several documented sources of error which may affect the accuracy of the ammonia measurement. One source of error with existing enzyme techniques is that ammonia, as a combination of the gaseous and ionic state, is generated by the deamination of endogenous amino acids in the sample as soon as the blood is withdrawn. Delays greater than 15 minutes before centrifuging of the sample have been reported as causing a clinically significant increase in measured ammoniacal concentrations. Other sources of error include variations in test strip or reagent consistency used to indicate analyte, inconsistencies in indicator sensing means, variations in homogeneity of ammonia distribution in the blood sample, and variation due to the background levels of ammonia gas in the laboratory environment at the time of actual specimen assay. For discussion of current ammoniacal concentration measurement techniques and devices, see:
Even assuming an accurate measurement, the time and expense associated with these types of analysis limit their repeatability during a given time period. The assay process can take 30 minutes or more once the sample is introduced into the analyzer. The expense involved with each blood sample includes the cost for the ammoniacal concentration assay as well as hospital staff time and expenses associated with withdrawing a blood sample, centrifuging the blood sample in a refrigerated centrifuge, and transporting the blood sample to the hospital's laboratory for assay. Given that these procedures are relatively expensive and labor intensive, blood ammoniacal concentration measurements are necessarily performed on an infrequent basis, typically several times per day. As such, trending, which would indicate the necessity for intervention where a patient's ammoniacal concentration level begins to rise but before a dangerous condition is reached, is not possible.
In view of the problems associated with existing blood ammoniacal concentration measurement techniques, a need exists for an approach which is more accurate, less expensive, and less time-consuming. Such an approach could consequently be performed more frequently allowing the practitioner to monitor trends in a patient's ammoniacal level and to provide more timely diagnosis and treatment.